This invention relates to semiconductor memory devices and more particularly to an MOS ROM which is electrically programmable.
Semiconductor memory devices which are nonvolatile have great utility in that the information stored is not lost when the power supply is removed. MOS ROMs provide storage of information which is permanently fixed upon manufacture by the gate level mask or moat mask as set forth in U.S. Pat. No. 3,541,543, assigned to Texas Instruments. Most calculators and microprocessor systems employ ROMs of this type to store a program consisting of a large number of instruction words. However, it is preferable to be able to program the ROM devices after manufacture instead of during manufacture so that all devices can be made the same and no unique masks are required and so that a specifically programmed device can be produced in a few minutes instead of requiring a cycle time of weeks. Various electrically programmable ROM devices have been developed as shown in U.S. Pat. No. 3,984,822, as well as in pending applications Ser. No. 754,144, Ser. No. 754,207, and Ser. No. 754,145, filed Dec. 27, 1976, and Ser. No. 1,095 and Ser. No. 1,097 filed Jan. 5, 1979, all assigned to Texas Instruments; these devices are double level polysilicon MOS ROMs having floating gates which are charged by injection of electrons from the channel. Other devices of this type have employed charge storage on a nitride-oxide interface. Electrically alterable ROMs have been developed as set forth in U.S. Pat. Nos. 3,881,180, issued Apr. 29, 1975, and 3,882,469, issued May 6, 1975, as well as application Ser. No. 644,982, filed Dec. 29, 1975, all by M. W. Gosney and assigned to Texas Instruments; the Gosney devices are floating gate cells with dual injection (both holes and electrons) so that the gates may be charged or discharged. Memory devices which are programmable by fusable links or by dielectric breakdown are shown in U.S. applications Ser. No. 990,550, filed Apr. 27, 1978, and Ser. No. 626, filed Jan. 2, 1979, assigned to Texas Instruments. However, even though successful products are marketed using these concepts, the prior cells have exhibited some undesirable characteristics such as large cell size, process incompatible with standard techniques, high voltages needed for programming, low process yields, low speed or other factors.
It is therefore the principal object of the invention to provide an improved electrically programmable floating gate type semiconductor memory cell. Another object is to provide an electrically programmable cell which is of small cell size when formed in a semiconductor integrated circuit. A further object is to provide an improved and simplified process for making electrically alterable memory cells in N-channel silicon gate technology.